The highest web speeds in paper machines are today up to an order of about 25 meters per second and slightly higher, but before long, a web running speed in the range of 25-40 meters per second (mps) will be commonly used. In such a case, a bottleneck for the runnability of a paper machine will be the dryer section, whose length with prior art multi-cylinder dryers would also become intolerably long. If it is imagined that a present day multi-cylinder dryer were used in a newsprint machine at a web speed of about 40 mps, it would include about 70 drying cylinders (.phi..apprxeq.1800 mm), and its length in the machine direction would be about 180 meters. In such a case, the dryer section would comprise about 15 separate wire groups and a corresponding number of draws over group gaps. It is probable that, in a speed range of 30-40 mps, the runnability of normal prior art multi-cylinder dryers is no longer even nearly satisfactory, but web breaks would occur quite often lowering the efficiency of the paper machine.
In a speed range of 30-40 mps and at higher speeds, the prior art multi-cylinder dryers would also become uneconomical because the cost of investment of an excessively long paper machine hall would become unreasonably high. It can be estimated that the cost of a paper machine hall is at present typically about 1 million FIM per meter in the machine direction.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web one from above and the other one from below against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders in order to maintain the web on the wire. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
It is known to those skilled in the art that if paper is dried one-sidedly, the result is a tendency of curling of the sheet. For example, when paper is dried by means of normal groups with single-wire draw from the side of its bottom face only, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
It is known in the prior art to use various ventilation/impingement-drying/through-drying units for evaporation drying of a paper web, which units have been employed in particular in the drying of tissue paper. With respect to this prior art, reference is made, by way of example, to the following patent literature: U.S. Pat. Nos. 3,301,746, 3,418,723, 3,447,247, 3,541,697, 3,956,832 and 4,033,048, Canadian Patent No. 2,061,976, West German Patent Application Nos. DE-A-22 12 209 (corresponding to U.S. Pat. No. 3,816,941) and DE-A-23 64 346 (corresponding to U.S. Pat. No. 4,033,049), European Patent Application No. EP-A2-0 427 218, Finnish Patent Nos. 83,679, 57,457 (corresponding to Swedish Patent Application No. 7503134-4) and 87,669 (corresponding to U.S. Pat. No. 5,383,288), and Finnish Patent Application No. 931263 (corresponding to U.S. Pat. No. 5,495,678 and European Patent Application No. 0 620 313-A1).
Additionally with respect to the prior art related to the present invention, reference is made to the current assignee's Finnish Patent Application Nos. 913648 (corresponding to U.S. Pat. No. 5,279,050), 940992 (corresponding to U.S. Pat. No. 5,539,999), 941392 (corresponding to U.S. Pat. No. 5,553,393) and 951746. Different dryer section concepts and geometries and impingement-drying units are described in the Finnish Patent Applications mentioned above.
Of the current assignee's Finnish patent applications mentioned above, FI 951746 is most closely related to the present invention. This Finnish patent application describes a dryer-section concept of a paper/board machine, wherein the dryer section comprises a number of drying cylinder groups, which comprise exclusively a single-wire draw, on whose support the web is guided so that it meanders as loop-shaped from a suction cylinder onto a drying cylinder and from the drying cylinder onto a second suction cylinder and therefrom further onto a second drying cylinder. Also, the wire supported the web is guided in connection with the drying cylinders so that the web is placed against the face of the drying cylinder and the wire is placed outside. In connection with the suction cylinders/suction rolls, the wire supporting the web is guided so that, by means of suction, the web is kept in contact with the wire draw, and guided into connection with the second heated drying cylinder. In the dryer-section concept, the web is passed from one drying cylinder group into the next dryer-section group and so on through the dryer section.
In FI 951746, it has been considered a novel feature in the context mentioned above that at least some of the drying cylinders include impingement-drying units or equivalent in connection with them, through which units a heated medium, preferably air or steam, is passed through the wire into connection with the web so as to produce a two-sided drying effect and to increase the drying capacity. Also, the impingement units are situated in the end of the dryer section in an area when the web has such a dry solids content that by means of impingement-drying, it is possible to affect the curling of the web and to prevent it. The impingement-drying units are mounted above contact drying cylinders with normal diameter (.phi. usually about 1800 mm) and it is suggested that the impingement-drying units be placed in the initial end, in the middle area and in the final end of the dryer section. Owing to the matters stated above, the drying concept in FI 951746 cannot achieve the objectives that the present invention aims to achieve, above all because the area of effect of the impingement units cannot be made wide enough if drying cylinders with normal diameters are used and the effect of the impingement-drying units cannot be concentrated on the process stage where their effect would be optimal in view of the objectives of the present invention.